Glossary of flood protection terms |
| Word |
Definition |
| Acre foot |
The volume of water required to
cover one acre of one foot (approximately 325,000 gallons). |
| Bank protection |
Bank protection involves any action
by the District to streambanks that are eroding (repair) as well
as preventative erosion protection. The District implements streambank
protection when the problem (1) causes or could cause significant
damage to a property or adjacent property, (2) is a public safety
concern, (3) negatively affects transportation or recreational use,
(4) negatively affects water quality, or (5) negatively affects
riparian habitat. Bank protection stabilizes a channel bank using
rock, riprap, concrete, soft materials, vegetation, or a combination
of materials or methods. Bank protection can also include preventative
maintenance to ensure that banks do not erode in the future. This
new work is considered routine maintenance because it is either
restoring the flood protection function of a modified channel or
it is repairing a natural bank to its approximate condition prior
to becoming an erosion problem. |
| Bank repair |
Maintenance of existing bank protection
structures with in-kind, in-place materials. This type of maintenance
occurs when such structures fail. |
| Bypass channel |
A flood protection facility through
which a portion of a stream's flow is diverted from one point and
reintroduced into the stream at the downstream end of the bypass
channel. Bypass channels can be used during the construction or
maintenance process. Permanent bypass channels can also be designed
to accommodate flood flows. |
| California
Environmental Quality Act (CEQA) |
The CEQA is California Public Resources
Code Sections 21000 et seq. CEQA establishes a duty for public agencies
to avoid or minimize environmental damage where feasible, recognizing
that a public agency has an obligation to balance a variety of public
objectives, including economic, environmental, and social factors.
CEQA is intended to facilitate the disclosure of the significant
environmental effects of proposed activities to decision-makers
and the public, the identification of ways to avoid or reduce environmental
damage, and the prevention of environmental damage by requiring
the implementation of feasible alternatives or mitigation measures. |
| Channel erosion |
Includes the processes of streambank
erosion, streambed scour, and degradation. |
| CFS or cubic
feet per second |
A
rate of flow that would fill a container of one cubic foot size,
that is about 7-1/2 gallons, in one second. |
| Confluence |
A meeting of two or more streams
or rivers. |
| Culvert |
Any covered structure not classified
as a bridge which conveys a waterway under a road or other paved
area. |
| Degradation |
Process of a channel lowering its
elevation through increased erosion, channel bed scour, or down-cutting.
A type of fluvial geomorphic instability. |
| Design capacity |
An engineering term used to describe
the amount of water that a modified channel was designed to convey.
Generally, the design capacity for improved District facilities
is to accommodate the 1 percent or 100-year flood. This is the level
of protection. Capacity is in CFS or Q. See "Flood Capacity." |
| Design flow |
The magnitude of stream flow that
is used in design of channel improvements and structures across
the channels. |
| Drainage
area |
Area that drains into a body of
water such as a stream or a reservoir. |
| Drop structure |
A structure designed to convey flows
over a vertical distance from a higher to a lower elevation. |
| D/S |
Abbreviation for "downstream
of". |
| Easement |
Right of way granted by a property
owner to the Santa Clara Valley Water District for an expressed
purpose of either constructing flood and erosion control improvements
or maintaining the channel. The property owner retains the ownership
rights on the area under the easement but his use of the area is
restricted only by those uses which are compatible with the District
uses. |
| Environmental
Impact Report (EIR) |
A
detailed statement prepared under CEQA describing and analyzing
the significant environmental impacts of a project and discussing
ways to mitigate or avoid the effects. |
| Erosion |
The detachment and movement of soil
or rock fragments by water, wind, ice, gravity, or extreme sun or
heat. |
| FEMA |
Federal Emergency Management Agency.
Has authority over federal flood insurance program and publishes
the 100-year flood map. |
| Fish ladders |
Artificial, stepped pools to enable
fish traveling upstream, against the flow of water, to span a large
vertical distance in a series of gradual steps. Used at dams or
other in-stream barriers. |
| Flood |
The temporary inundation of lands
normally dry; any waters escaping from a creek or river. |
| Flood capacity |
The capacity of a channel to carry
calculated flood flows. Capacity is dependent on cross-sectional
area and frictional components (e.g., channel vegetation). |
| Flood protection
project |
A project that affects the flood
conveyance capacity or flood management behavior of the system,
usually designed to reduce flooding hazards. |
| Flood control
zone |
A zone established by the SCVWD
for the purpose of limiting use of tax funds available from a certain
area (zone) for the improvements and services in that particular
area only. There are five flood control zones in Santa Clara County. |
| Flood plain |
Low
lying areas that are flooded during high flows in a channel. |
| Floodwall |
A wall constructed adjoining channel
to prevent flooding of the surroundings areas. |
| Freeboard |
Vertical
distance between the top of an embankment adjoining a channel and
the water level in the channel. |
| Habitat |
The specific area or environment
in which a particular type of plant or animal lives. To be complete,
an organism's habitat must provide all of the basic requirements
of life for that organism. |
| Hard structures |
A type of bank protection structure
incorporating rock, riprap, sack concrete, gabion baskets and mattresses,
or concrete. These structures are inert and rigid. |
| Hydrology |
The
science dealing with the origin, distribution and circulation of
waters of the earth such as rainfall, streamflow, infiltration,
evaporation, and groundwater storage. |
| Invasive
species |
A subcategory of nonnative plants
that aggressively invade natural plant communities and displace
native plants or less aggressive weedy plants. Examples of invasive
species in Santa Clara County wetland and riparian areas are broad-leaf
peppergrass (Lepidium latifolium) and giant reed (Arundo donax). |
| Invert |
A creek or channel bottom. |
| Levee |
An embankment constructed to prevent
a river or stream from flooding adjacent lands. |
| Low flow
channel |
That section of stream which carries
the more frequent, periodic streamflows. |
| Mean higher
high water (MHHW) |
The average height of the higher
of the two high tides in the San Francisco Bay. |
| Mean high water (MHW) |
Mean High Water. In San Francisco
Bay, there are two high tides each day, usually with different elevations.
Mean High Water is defined as the average height of both of these
two tides. |
| Mean sea
level (MSL) |
A
level midway between mean high and mean low tides in an open sea. |
| Modified
channel |
A waterway in which engineered alterations
have occurred to improve the passage of flood flows or to provide
drainage. This includes straightening (or channelization), containing
a watercourse within constructed banks or levees, or lining banks
with concrete, riprap, gabions, or sack concrete. |
| Modified
natural channel |
A watercourse which has had improvements
such as bank protection (e.g., gabions, rip rap, other revetments)
and selected areas of historical channelization (e.g., widening,
straightening) and/or other capacity or passage improvements. |
| Natural channel |
A watercourse without any significant
improvements or modifications and very little evidence of historical
alterations. |
| Nonnative
vegetation |
Any vegetation which, under natural
conditions, does not originate within the ecosystem in which it
is found. |
| Reach |
The smallest subdivision of a drainage
system consisting of a uniform length of channel or a discrete portion
of a channel. |
| Restoration |
The reestablishment of the structure
and function of ecosystems. Ecological restoration is the process
of returning an ecosystem as closely as possible to predisturbance
conditions and functions. Implicit in this definition is that ecosystems
are naturally dynamic. It is therefore not possible to recreate
a system exactly. The restoration process reestablishes the general
structure, function, and dynamic but self-sustaining behavior of
the ecosystem. |
| Riparian |
Located along the edge of a channel,
generally on the floodplain. Characterized by access to and influence
of the channel, but not in it. |
| Riprap |
Loose rock or concrete of varying
size, typically brought to a site. Used to protect channel banks
from scouring forces. |
| Runoff (surface) |
The flow of water across the land
surface and in stream channels. Occurs only after the local storage
capacity of the landscape has been exceeded and includes both overland
flow and streamflow. |
| Scour |
The clearing and digging action
of flowing air or water, especially the downward erosion caused
by stream water in removing material (e.g., soil, rocks) from a
channel bed or bank or around in-channel structures. |
| Sediment |
Solid material, both mineral and
organic, that settles to the bottom of channels, canals, percolation
ponds, or behind dams. |
| Sediment
removal |
The act of removing sediment deposited
within a stream. Typically, sediment is removed when it reduces
capacity. |
| Sheetflooding |
A shallow (less than a foot) flooding
caused by inadequate drainage systems. |
| Station |
Station is a standard channel location
system used by the SCVWD that gives the distance from the downstream
limit of jurisdiction (usually San Francisco Bay), or, for a tributary
creek, from where it branches off of the main channel. Distance
is measured in feet, with each "station" representing
100 feet. For example, station 43+56 would be a point 4,356 feet
upstream from the mouth of the channel. |
| Stream
Maintenance Program |
The Stream
Maintenance Program will provide long-term guidance to the District
to effectively implement routine stream maintenance projects in
a cost-effective and environmentally-sensitive manner. The Stream
Maintenance Program is to be codified in a process and policy document
that can be adopted by the District and utilized in obtaining long-term
permits from regulatory agencies. The Stream Maintenance Program
includes specific measures, protocols, and monitoring and reporting
requirements to ensure that routine stream maintenance projects
are implemented in an effective, cost-sensitive, and environmentally-sensitive
manner. |
| Streambed |
The part of a stream over which
a column of water moves. |
| U/S |
Abbreviation for "upstream
of" |
| Vegetation
management |
Removal of vegetation in and adjacent
to creeks to maintain the ability of channels to function as flood
protection facilities. In addition, vegetation is removed to meet
local fire code requirements and to reduce combustible weeds and
grasses on property adjacent to the streams within the District's
jurisdiction. The control of invasive nonnative vegetation is another
purpose for which the District undertakes vegetation control. Vegetation
management can be accomplished through mowing, discing, hand clearing,
or herbicide applications (depending on the environmental conditions
of the site). |
| Velocity |
Speed
with which water should flow in a channel. It depends on several
factors, such as slope, smoothness and uniformity of channel, area
of flow and wetted perimeter. |
| Watershed |
A geographic area from which water
is drained by a river and its tributaries to a common outlet. A
ridge or drainage divide separates a watershed from adjacent watersheds. |
| 1% Flood |
Refers to a flood of a magnitude
that has an estimated probability of 1 in 100 of occurring in any
given year. Technically more precise way of referring to the "100-year
flood". Generally, 1%, 2%, 10% events refer to levels of flood
flows with an expected recurrence of 100, 50, and 10 years respectively. |
| 100-Year
Flood |
Flood of a magnitude with an expected
recurrence of once in 100 years. Synonymous with 1% flood. |
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